Using the GNU Privacy Guard: Automated signature checking

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#### 4.5.1 Automated signature checking

It is very important to understand the semantics used with signature verification. Checking a signature is not as simple as it may sound and so the operation is a bit complicated. In most cases it is required to look at several status lines. Here is a table of all cases a signed message may have:

The signature is valid

  This does mean that the signature has been successfully verified, the certificates are all sane. However there are two subcases with important information: One of the certificates may have expired or a signature of a message itself as expired. It is a sound practise to consider such a signature still as valid but additional information should be displayed. Depending on the subcase `gpgsm` will issue these status codes:

  signature valid and nothing did expire

    `GOODSIG`, `VALIDSIG`, `TRUST_FULLY`

  signature valid but at least one certificate has expired

    `EXPKEYSIG`, `VALIDSIG`, `TRUST_FULLY`

  signature valid but expired

    `EXPSIG`, `VALIDSIG`, `TRUST_FULLY` Note, that this case is currently not implemented.

The signature is invalid

  This means that the signature verification failed (this is an indication of af a transfer error, a program error or tampering with the message). `gpgsm` issues one of these status codes sequences:

  ``BADSIG``
  ``GOODSIG`, `VALIDSIG` `TRUST_NEVER``
Error verifying a signature

  For some reason the signature could not be verified, i.e. it cannot be decided whether the signature is valid or invalid. A common reason for this is a missing certificate.